Process for metal recovery

ABSTRACT

Metals, particularly precious metals, embedded in or otherwise confined by enzyme-degradable organic media as in photographic film or printed circuits are recovered in the form of metal or metal compounds by breakdown of said media primarily by enzymes.

United States Patent Bakker July 24, 1973 PROCESS FOR METAL RECOVERY [56] References Cited [75] Inventor: Lubertus Bakker, South Euclid, I OTHER PUBLICATIONS Ohio Sramek, Chemical Abstracts, Vol. 59, 1963, p. [73] Assignee: Bakker Development Corp., 4715g Cleveland, Ohio Primary Examiner-Herbert T. Carter Filed? J y 14, 1971 Attorney-McCoy, Greene & Howell [21] A 1. No.: 162 650 pp 57 ABSTRACT Metals, particularly precious metals, embedded in or [52] US. Cl 75/97, 745zlll48 442235348, otherwise confined by enzyme degradable organic [51] Int Cl czzb 7/00 media as in photographic film or printed circuits are re- [58] Fie'ld 423/34 covered in the form of metal or metal compounds by 423/43 breakdown of said media primarily by enzymes.

4 Claims, No Drawings PROCESS FOR METAL RECOVERY BACKGROUND OF THE INVENTION The commonest method for the recovery of silver from exposed photographic film or photographic prints is to burn the film and/or prints in an incinerator and to recover the silver by refining the resulting ash. Not only is the base film and/or paper completely destroyed by this method, but this method requires the maintenance of a critical temperature range of combustion to assure an adequate silver recovery rate while usually resulting in the production of excessive smoke and fly ash. There is also sufficient entrainment and/or vaporization of the desired silver compounds in the system under the temperature and turbulence conditions of incineration that the silver grain loading in the exhaust gases usually exceeds the limits permitted by law.

Furthermore, the heat generated by burning film is so great that the average incinerator can operate at only about one-fourth to one-third of its rated capacity to avoid generation of temperatures which would result in destruction of the incinerator and in the emission of flames and sparks together with the aforementioned smoke, fly ash and silver compounds. At one-third of its rated capacity an incinerator must burn for 3 days to produce a full day of rated production at controlled temperatures. This inefficiency is costly;

SUMMARY oF THE INVENTION This invention is directed specifically to the recovery of silver from photographic film by the use of proteases, also known as proteinases or proteolytic enzymes, for the destruction of the film gelatin which holds the silver compounds. Obviously, this invention more generally relates to the recovery of precious metals and compounds of precious metals, particularly metal salts, from proteinaceous materials and most generally relates to the recovery of metals and metal compounds from emzyme-degradable organic binder materials by the use of active enzymes to decompose said binder materials. In all cases the recovered metals and/or metal compounds can be reduced and/or refined to pure metals by methods well known in the art.

This invention makes possible the recovery of silver from photographic film without destruction of the film base. This is not only an economic advantage but avoids the problems of air pollution inherent in a burning process. The recovered stripped film base can be sometimes used as is or can be ground up and reused for molding or extruding or can be dissolved in an organic solvent and completely reconstituted for casting new film. It is similarly possible with the same advantages of this invention to recover silver, gold and/or platinum from obsolete or defective printed circuits.

The method of this invention is enhanced in its application to photographic film by the use of an alkaline aqueous solution to fragmentize the silver-containing gelatin on the film base before attacking the resulting protein fragments with proteases. Shredding the film also improves the efficiency of the method of this invention, particularly where the film pieces are stuck together.

The principal object of this invention is to provide a novel method for the recovery of metals and metal compounds, especially precious metals and their compounds, from enzyme-degradable media. Another object is to provide an efficient method for separating metals and metal compounds from proteinaceous materials. Another object is to provide a method for recovering silver and silver compounds from photographic film without burning the film base. These ob- 5 jects as well as others which are obvious from the following description are satisfied by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of this invention comprise the use of proteases to hydrolyze, digest, depolymerize or otherwise degrade or decompose proteinaceous material, especially animal protein such as in gelatin, thereby releasing metals and/or metal compounds embedded in or otherwise retained by the proteinaceous material.

Proteases, also referred to as proteinases or proteolytic enzymes, by definition are enzyme compounds which hydrolyze, digest, depolymerize or otherwise degrade or decompose protein. Papain is the preferred enzyme because of its thermal stability, water solubility and activity over a wide pH range as well as over a broad range of protein compositions. However, other typical examples of proteases are pepsin, rennin, trypsin, chymotrypsin, pankrin, enterokinase, chymopapain, ficin, bromelin, B. subtilis proteinase, insulinase, Aspergillus proteinase, carboxypeptidase, protaminase, asparaginase, penicillin amidase, penicillinase, cerevase and rapidase.

While the protease in aqueous solution is the important ingredient for the purposes of this invention, commercial products can be used which include both active enzymes as well as extraneous inert matter such as plant leaves, rinds and the like. Also other enzymes can be present such as amylase, which destroys carbohydrates, or maltase, which breaks down carbohydrates to lower sugars. Cellulose-splitting enzymes are of particular value in systems wherein metal recovery is enhanced by the ability to break down paper.

As an example for specific application of this invention photographic film is immersed in an aqueous protease solution or a liquid protease under time, temperature and pH conditions sufficient to enable the protease to digest the gelatin on the film surface so that the silver and/or silver compounds which are a part of the film settle out of the solution as they are freed from the gelatin binder and the resulting stripped film base can be recovered.

The pH and temperature of the enzyme solution depends primarily on the optimum conditions for maximum activity of the proteolytic enzymes employed..

Preferably, the pH is in the range of about 4.0 to about 7.0, more preferably from about 4.5 to about 6.0, although for pepsin the pH can be as low as 1.5 and for some of the commercial enzyme concentrates of fungal or bacterial origin the pH can be as high as 9.0. The temperature range is preferably from about 30 to about C. or about to about 160 F., more preferably from about 35 to about 50 C. or about to about F.

The enzyme solution is preferably acidified with an organic acid such as acetic acid because such an acid even in very small quantities is more than adequate-to maintain the desired solution pH and because any metal'salts of such acids are easily decomposed to metal during refining of the metal product as by heating to decomposition.

The active proteolytic enzymes, particularly papain, can be employed as aqueous solutions with concentrations as low as 0.05 weight percent or they can be employed in a l percent pure state is they are in liquid form.

The time necessary for the method of this invention is finite but indeterminant. After photographic film has been immersed in a suitable enzyme solution for no more than one hour, the film can be observed to be losing its gelatin coating. The procedure is expedited by agitating the film and solution and/or mechanically wiping the film after the coating is loosened. It is also desirable to shred the film before treatment, especially when the film pieces are stuck together thereby making penetration by the enzyme solution difficult. Such shredding does not render the stripped film unmarketable because the plastic can still be reconstituted.

While the method of this invention is operable starting with scrap photographic film, it is preferable to start the protein breakdown of the gelatin by the use of an alkaline solution before the enzyme solution, thereby starting to strip the gelatin or other metal-containing proteinaceous matter from the film base. Any alkali metal hydroxide can be used. The alkaline concentration can be any concentration which will attack the gelatin. For example, 3 weight percent and weight percent sodium hydroxide solutions have been found to be effective, the more concentrated solution working significantly faster than the less concentrated solution. The temperature of the alkaline solution can be anything, ambient temperatures being satisfactory, but heat will generally make the system more active. Even with the alkaline pretreatment it is highly desirable to shred the scrap film starting material and to agitate the shredded scrap film during any treatment.

The film treatment in the alkaline solution should preferably be limited to a maximum of 2 hours and is generally complete in 30 to 40 minutes. Then the stripped film base is removed, and the solution is allowed to settle for at least minutes, preferably at least 30 minutes. During the settling time the stripped film base can be permitted to drain into the solution. After settling is complete, the alkaline solution is preferably siphoned off to prevent unnecessary loss of product, especially in case the alkaline solution is not reused.

The stripped film base should be spray washed such that the level of the recovered wash water is below the film. Then the wash water is added to the residue from the alkaline solution.

The residue from the alkaline solution is then mixed thoroughly and continuously with protease solution as described above, acidified if necessary, and heated to the desired temperature for a short period, generally no more than l or 2 hours, preferably about to minutes. The system is then again allowed to settle and the enzyme solution siphoned off. The resulting sludge contains at least 90 percent, usually at least 95 percent, of the metal in the starting film either as pure metal or a metal compound. The sludge can be filtered and refined by any of the methods known for producing pure metal. For example, the sludge can be heated at extreme temperatures to burn off any organic matter and- /or it can be reduced to metal by heating in a reducing (hydrogen) atmosphere at a sufficient temperature, e.g. about 300 C. or 575 F., to produce hydrogen bromide and hydrogen chloride as valuable by-products or it can be simply smelted to give the pure metal.

The following examples are illustrative of the work done to date and are not intended to limit this invention which is delineated in the appended claims.

EXAMPLE I A batch of 1.9 pounds of photographic X-ray acetate film, coated on both sides with gelatin and containing about 0.27 troy ounce of silver per pound of film, was immersed in 2 quarts of water containing 0.05 percent by weight of papain enzyme for 24 hours at 65 F. This temperature was used primarily for the study of temperature influence. The temperature of the water was then raised to 100 F. for another 48 hours after which essentially all of the silver had been removed from the film and the water was decanted leaving a residue which was dried at 175 F. for about 12 hours to form a tough hard black solid consisting of silver, silver salts, primarily silver chloride and silver bromide, and products of silver with the gelatin breakdown products. The solid product was heated for a time and at a temperature to destroy all organic matter, producing a tan fluffy residue which was then heated in a kiln at l,800 F. to produce a mixture of silver and silver bromide containing 0.46 troy ounce of silver. This was a silver yield of about weight percent.

EXAMPLE II A batch of 1.32 pounds of photographic X-ray acetate film strips, coated on both sides with gelatin and containing about 0.27 troy ounce of silver per pound of film, was immersed in 500 ml. of a 10 weight percent sodium hydroxide solution at ambient temperature. After 1 hour all the film was clear of gelatin and silver compounds. A second identical batch of 1.32 pounds of film strips was then immersed in the same sodium hydroxide solution at ambient temperature. After 1% hours all the film was clear of gelatin and silver compounds. The film strips were removed from the sodium hydroxide solution and rinsed with clear water. The sodium hydroxide solution was allowed to stand for about 12 hours after which a residue had settled. The dark, clear supernatant liquid was decanted. The residue was neutralized by washing four times with cold fresh water, the second batch of wash water containing some acetic acid. The wash water from the fourth batch was neutral.

The resulting residue was immersed in ml. of water containing 0.05 weight percent of papain enzyme. After the system had been maintained at 100 F. for 3 hours, the proteinaceous material in the residue had disappeared. The enzyme solution was then decanted from the remaining residue, which was washed once with cold fresh water which was in turn decanted.

The resulting black residue was dried in an oven at F. and turned into a tan fluffy solid. This fluffy solid was ground up, put in a ceramic boat and treated with hydrogen for several hours at about 550 F., producing 0.683 troy ounce of pure silver, a yield of 96 weight percent.

From the above experiments the best method of practicing this invention for the recovery of silver from photographic film comprises the following steps:

1. Shred the film from which the silver is to be recovered if the film pieces are stuck together.

2. Immerse the film base with agitation in an alkaline solution of sufficient strength to start decomposition and stripping of the film gelatin from the film base within a reasonable time.

3. Remove and wash the stripped film base for further processing.

4. Remove the alkaline solution and neutralize the resulting residue.

5. Treat the residue with an aqueous solution containing at least 0.05 weight percent of a suitable protease such as papain for a time sufficient to destroy all protein in the residue.

6. Separate the resulting residue from the protease solution and refine the residue for silver recovery.

There is presently paper-based film which can be treated by the method of this invention for recovery of silver compounds, but it is preferable that such film be treated, first, with the protease solution and then with any alkaline solution.

I claim:

l. A method for the separation of silver and silver compounds from photographic film which comprises a plastic film base and proteinaceous binding media to hold any silver and silver compounds, said method comprising (1) subjecting said film to a first aqueous solution of an alkali metal hydroxide, said solution being of sufficient strength to start decomposition of said proteinaceous binding media within 2 hours, (2) separating said film base from the resulting residue of other film components, (3) separating said residue from said first aqueous solution and neutralizing said residue, (4) subjecting said residue to a second aqueous solution containing as the essential ingredient at least 0.05 weight percent of a protease, said second aqueous solution having a pH in the range of 4 to 7, for a time and at a temperature sufficient to destroy essentially all of said proteinaceous binding media and (5) separating the resulting residue from said second aqueous solution.

2. A method in accordance with claim 1 wherein said photographic film is shredded prior to being subjected to said first aqueous solution.

3. A method in accordance with claim 2 wherein said second aqueous solution is heated to temperatures in the range of about 30 to about C.

4. A method in accordance with claim 1 wherein said protease is papain. 

2. A method in accordance with claim 1 wherein said photographic film is shredded prior to being subjected to said first aqueous solution.
 3. A method in accordance with claim 2 wherein said second aqueous solution is heated to temperatures in the range of about 30* to about 70* C.
 4. A method in accordance with claim 1 wherein said protease is papain. 